I couldn't find this anywhere on the internet, so I figured I would add to the knowledge out there: If you have Snow Leopard and an Active Directory infrastructure that you have extended the schema on (or have a dual directory structure) and want to enforce passwords on screen saver unlock, it's not easy. Set up the appropriate computer group. On the permissions for the group, select "Details" and add a "com.apple.screensaver" item (any screensaver will do.)
Edit the item, and under "Always" add a New Key of "askForPassword"
Set that to a Type of Integer and set the integer to 1.
That's it. You're done, and you can stop googling for "Snow Leopard Active Directory Enforce Screensaver" or "OS X Screen Saver Active Directory."